1. Field of the Invention
The present invention relates to an automated pattern inspection system, and more particularly to an automated defective pattern inspection system for detecting and evaluating defects in a digitized image pattern such as circular, square and regular polygonal patterns by comparing a squared boundary length derived from a measured boundary length of the pattern with another squared boundary length obtained from a measured area of the same pattern.
2. Prior Art
A requirement exists for the detection and evaluation of defects in a digitized image pattern automatically and within a short period of time. For example, in the case of a character recognition system wherein the character recognition rate is improved by detecting and correcting the omission or ambiguity of a character pattern, or in the case of a printed circuit board or integrated circuit photomask, a small part of the image pattern of which is detected as a digitized pattern by means of an image sensor or television camera, there is a need to judge whether the digitized pattern is fatally defective or not.
Hitherto, there has been proposed an apparatus for eliminating and extracting a small defective portion in a pattern as in Japanese Patent Application No. 97122/71 (Japanese Unexamined Patent Application Publication No. 61030/73). In this pattern inspection apparatus, a portion having either one of two conditions of a digitized pattern is first expanded and thereafter compressed or first compressed and thereafter expanded in such a manner that a small portion (the defective portion) contained in the pattern is eliminated. After this elimination process, a pattern obtained by the elimination of the small portion is compared with the original pattern so as to extract the small portion in the original pattern. This apparatus has disadvantages in that there is the possibility that an irregular boundary of the object pattern caused by sampling errors when the pattern is sampled will be extracted as a defect and in that a large size filter is required in order to detect a relatively large defect thereby complicating the construction of the apparatus.
With the above in mind, the inventors have proposed a defect recognition apparatus in Japanese Patent Application No. 9558/76 (Japanese Unexamined Patent Application Publication No. 93248/77), based upon the principle that the total boundary length of a pattern is prolonged if the pattern has a defect therein. In this apparatus, a boundary length calculated from the actual area of the pattern is compared with a boundary length obtained by measuring the boundary itself in order to evaluate the size of a defect. In this prior art apparatus, picture elements of an image pattern along the boundary in the X or Y direction are detected so as to count the number of these picture elements, and at the same time picture elements along the boundary in both of the X and Y directions are detected so as to count the number of these picture elements. The boundary length of the image pattern is computed from these two count values and thereafter compared with a second boundary length calculated from the actual area of the pattern to judge whether or not the pattern contains a defect. According to this apparatus, not only a relatively small defect but also a relatively large defect can be recognized and in addition this recognition can be performed by a simple arrangement.
In this apparatus, however, it is necessary to perform square root operation in either of the above mentioned computations for the boundary lengths, and it takes an extremely long computing time to perform the square root operation. Accordingly, it takes an extremely long time to obtain these boundary lengths.
In other words, if substantially the same defect evaluation can be realized without the square root computation, the evaluation can be completed within a shortened computing time. This leads to a high speed defect inspection apparatus.